(a) Field of the Invention
This invention relates to shape-memory alloy (SMA) actuators. In particular, this invention relates to SMA actuators, especially miniaturizable SMA actuators, with improved temperature control for faster response and extended working life.
(b) Description of Related Art
A class of materials was discovered in the 1950s that exhibit what is known as the shape memory effect. See, for example, K. Otsuka, C. M. Wayman, “Shape Memory Materials”, Cambridge University Press, Cambridge, England, 1998, ISBN 0-521-44487X. These materials exhibit a thermoelastic martensite transformation; i.e. they are pliable below a certain transition temperature because the material is in its martensite phase and can be easily deformed. When their temperature is raised above the transition temperature the material reverts to its austenite phase and its previous shape, generating a large force as it does so. Example of such materials are approximately 50:50 atom percent titanium-nickel (TiNi) alloys, optionally containing small quantities of other metals to provide improved stability or to alter the martensite-austenite transition temperatures; and these can be formulated and treated to exhibit the shape memory effect. Other such alloys include Cu/Al/Ni and Cu/Al/Zn alloys, sometimes known as β-brasses. Such alloys are generically referred to as shape memory alloys (SMA) and are commercially available from a number of sources in wire form, with diameters from as low as 37 μm to 1 mm or greater. See, for example, Dynalloy Corp., “Technical Characteristics of Flexinol Actuator Wires”, Technical Information Pamphlet, Dynalloy Corp., Irvine Calif., USA.
SMA wires are wires of shape memory alloy that are treated such that they can be easily stretched along their longitudinal axis while in the martensite phase, thus re-arranging their atomic crystalline structure. Once stretched they remain that way until they are heated above their austenite transition temperature, at which point the crystalline structure is restored to its original (remembered) austenite configuration. This reversion not only returns the wire to its original length, but also generates a large force, typically on the order of 50 Kgf/mm2 cross-sectional area, depending on the alloy and its treatment. Because of the large available force per cross-sectional area, SMA wires are normally produced in small diameters. For example, a 100 μm diameter wire can deliver about 250 g of force. To obtain more force, thicker wires or multiple wires are required.
Although SMAs have been known since 1951, they have found limited commercial actuator applications due to some inherent limitations in the physical processes which create the shape memory properties. This lack of commercial applications is due to a combination of the following factors:
(1) Limited Displacement
A TiNi SMA wire can contract by at most 8% of its length during the thermoelastic martensite to austenite transition. However, it can only sustain a few cycles at this strain level before it fails. For a reasonable cycle life, the maximum strain is in the 3-5% range. As an example, for an actuator with reasonable cycle life, it requires over 25 cm of SMA wire to produce 1 cm of movement.
(2) Minimum Bend Radius
An obvious solution to packaging long lengths of SMA into small spaces is to use some kind of pulley system. Unfortunately SMA wires can be damaged if they are routed around sharp bends. Typically an SMA wire should not be bent around a radius less than fifty times the wire diameter. As an example, a 250 μm diameter wire has a recommended minimum bending radius of approximately 1.25 cm for high cycle life. It should be noted that the term “minimum bending radius” as used here means the minimum radius within which an SMA wire can be bent and still be capable of repeated austenite-martensite cycling without damage. The addition of a large number of small pulleys makes a system mechanically complex, eliminating one of the attractions of using SMA in the first place. Also the minimum bend radius requirement places a lower limit on actuator size.
(3) Cycle Time
An SMA wire is normally resistively heated by passing an electric current through it. The wire then has to cool below its Ms temperature before it can be stretched back to its starting position. If this cooling is achieved by convection in still air, then it can take many seconds before the actuator can be used again. The 250 μm wire discussed above has a best cycle time of about 5 seconds or more in free air. Thus, as an example, Stiquito, an SMA powered walking insect [J. M. Conrad, J. W. Mills, “Stiquito: Advanced Experiments with a Simple and Inexpensive Robot”, IEEE Computer Society Press, Los Alamitos Calif., USA, ISBN 0-8186-7408-3] achieves a walking speed of only 3-10 cm/min. Since the rate of cooling depends on the ratio of the surface area of the wire to its volume, changes in wire diameter affect the cycle time, with smaller wires having shorter cycle times.
The problems of cycle time are exacerbated when the SMA actuator is subjected to repeated on-off cycling, such as if it were used in a Stiquito or similar toy or in another environment where the actuator is constantly cycled. Then, the air and any other components around the SMA elements may well become heated above external ambient temperature, resulting in a reduced ability of the SMA elements to release heat and cool to the martensitic state.
Working life (number of cycles) can also be adversely affected by the inability to control cooling, as rapid heating to achieve fill contraction can often result in the temperature of the SMA wire considerably exceeding the Af temperature, particularly over the central portion of the wire; and such repeated large temperature excursions cause fatigue in the wire and loss of working life.
To overcome these limitations designers of SMA based actuators have typically used long straight wires or coils. See, for example, M. Hashimoto, M. Takeda, H. Sagawa, I. Chiba, K. Sato, “Application of Shape Memory Alloy to Robotic Actuators”, J. Robotic Systems, 2(1), 3-25 (1985); K. Kuribayashi, “A New Actuator of a Joint Mechanism using TiNi Alloy Wire”, Int. J. Robotics, 4(4), 47-58 (1986); K. Ikuta, “Micro/Miniature Shape Memory Alloy Actuator”, IEEE Robotics and Automation, 3, 2151-2161 (1990); and K. Ikuta, M. Tsukamoto, S. Hirose, “Shape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscope”, Proc. IEEE Int. Conf. on Robotics and Information, 427-430 (1988). Clearly, in many applications, especially where miniaturization is desired, it is impractical to use long straight wires. Coils, although greatly increasing the stroke delivered, are bulky and significantly decrease the available force (the force is proportional to the sine of the pitch angle—the angle between the axis of the coil as a whole and the axis of a single turn of the coil—and that may be as low as a few degrees); and, to compensate for the drop in force, thicker wires are used which reduce the responsiveness of the resulting actuator, making it unsuitable for many applications.
Other mechanisms commonly used to mechanically amplify the available displacement, such as those disclosed in D. Grant, V. Hayward, “Variable Control Structure of Shape Memory Alloy Actuators”, IEEE Control Systems, 17(3), 80-88 (1997) and in U.S. Pat. No. 4,806,815, suffer from the same limitation on available force, again leading to the requirement for thicker wires and the attendant problems with cycle time.
A common method of heating SMA actuators to their transition temperature is pulse width modulation (PWM). In this scheme, a fixed voltage is applied for a percentage of a pre-set period. As the percentage on-time to off-time in a single period (referred to as the duty cycle) is changed, the aggregate amount of power delivered to the SMA can be controlled. This scheme is popular because of the ease with which it can be implemented in digital systems, where a single transistor is all that is required to drive an actuator, obviating the need for digital-to-analog conversion and the associated amplifiers.
PWM control is particularly attractive because many commercial micro-controllers contain built-in hardware for generating PWM signals, reducing the computational overhead on the controller; also, PWM output is often used in sound chips (such as those used in “talking” greeting cards and the like) as an inexpensive D-A conversion mechanism, making these low cost chips suitable as controllers for SMA actuators of this invention. In some applications, full PWM control may not be required, and an inexpensive timer chip could be used to generate the required digital signals. Also, PWM control reduces average current draw when a temperature signal is available, because no current limiting resistor is needed to prevent overheating the SMA element. Also, because current flow in an SMA wire tends (as with all solid conductors) to be concentrated at the surface of the wire, there is the risk of “hot-spots” and uneven heat distribution, reducing the life of the wire. Pulsing the activating voltage allows for thermal conduction in the SMA wire to lead to more even heat distribution. Further, in a conventional DC control system, the SMA current is effectively constant and relatively low, because it is determined by the current-limiting resistor, the value of which is chosen to avoid overheating of the SMA element once it is fully contracted. In a PWM or pulsed scheme with resistance feedback, a high duty cycle can be used to heat the SMA element initially, leading to rapid initial movement. The duty cycle can be reduced when the SMA element reaches the desired position, supplying only enough power to maintain the SMA element in the desired state.
The transition from the martensite (low temperature) phase to the austenite (high temperature) phase in SMAs does not happen instantaneously at a specific temperature but rather progresses incrementally over a temperature range. FIG. 1 shows the relationship between displacement and temperature for a typical SMA wire that has been placed under tensile stress and extended in its martensitic state, contracting on heating and conversion to the austenitic state, and re-extending as it cools and reverts to the martensitic state under the tensile stress. FIG. 1 shows the austenite start As and austenite finish Af temperatures, as well as the martensite start and finish temperatures Ms and Mf, respectively. In the temperature range indicated by ΔT the alloy consists of a mixture of austenite and martensite. As can be seen, substantially no change in length occurs below As, and substantially no further change in length occurs above Af, as the SMA is heated. Similarly, on cooling substantially no change in length occurs above Ms, and substantially no further change in length occurs below Mf; however, there is typically substantial hysteresis in the length-temperature curve. Also, maximum contraction of an SMA wire requires heating the wire to a temperature above Af, and maximum re-extension requires cooling to a temperature below Ms. This means that in practice such wires are frequently desired to be operated over a range of well below Ms to well above Af to achieve maximum contraction/re-extension.
The disclosures of all documents cited in this section and elsewhere in this application are incorporated by reference into this application.
It would be desirable to develop SMA actuators with improved temperature control for faster response (lower cycle time) and extended working life (greater number of cycles achievable).